This in vivo study was conducted to evaluate the effect of supplementation with different rates of euglena (Euglena gracilis) on nutrient intake, digestibility, nitrogen balance and rumen fermentation. Four rumen cannulated Corriedale wethers sheep with an average body weight of 44.25±3.86kg were arranged in a 4×4 Latin square design and fed a basal diet of Guinea grass (Panicum maximum) hay and concentrate mixture at the maintenance level with four different rates of euglena (0, 50, 100 and 150g/kgDM intake). The experiment was conducted over 80days in four 20day periods that consisted of 14days of acclimatization, 5days of measurement and 1 more day for rumen liquor sample collection. The data were subjected to polynomial regression analysis. Dry matter (DM), organic matter (OM), acid detergent fibre (ADF) and gross energy (GE) intake increased linearly (P<0.001) and quadratically (P=0.002) with increasing concentrations of euglena. Similarly, crude protein (CP) intake was increased linearly (P<0.001). Dry matter, OM, NDF, ADF and GE digestibility were not affected by supplementation of euglena (P>0.11) while apparent CP digestibility increased linearly (P=0.009). As a result, protein retention (g/d) was increased linearly (P<0.001) and quadratically (P=0.017) with increasing concentrations of euglena. Ruminal NH3-N concentration increased (linear, P<0.001) while ruminal protozoa population reduced linear, quadratic and cubic (P<0.008) with increasing doses of Euglena. Euglena supplementation at different concentration did not change (P>0.23) the total volatile fatty acid (VFA) concentration and the molar proportions of acetate, propionate, butyrate and the acetate: propionate ratio. The finding of this study indicated that the addition of euglena increased nutrient intake without affecting total tract digestibility. It has been also demonstrated that addition of euglena at a dose of 150g/kgDM improved CP retention by 31%, which may be associated with increased CP digestibility and efficiency of utilization. Thus, euglena supplementation up to 150g/kg DM of the diet could be a possible option for substitution of protein and energy sources.